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Ar diffusion and solubility measurements in plagioclases using the ultra-violet laser depth-profiling technique

By
Jo-Anne Wartho
Jo-Anne Wartho
1
Department of Earth Sciences, Open University, Walton Hall, Milton Keynes MK7 6AA, UK
2
Present address: School of Earth and Space Exploration, Arizona State University, PO Box 871404, Tempe, AZ 85287-1404, USA
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Simon P. Kelley
Simon P. Kelley
1
Department of Earth Sciences, Open University, Walton Hall, Milton Keynes MK7 6AA, UK
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Stephen C. Elphick
Stephen C. Elphick
3
School of Geosciences, Grant Institute, West Mains Road, Edinburgh EH9 3JW, UK
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Published:
January 01, 2014

Abstract

We describe the first direct measurements of Ar diffusion and solubility in plagioclases using ultra-violet (UV) laser ablation depth-profiling and noble gas mass spectrometer analyses of experimentally treated (599–1000 °C, 50–200 MPa of Ar) crystal fragments of labradorite and oligoclase. Labradorite 40Ar gain diffusion profiles were measured, yielding an activation energy of 26.72±4.58 kcal mol−1 (118.0±19.16 kJ mol−1) and a frequency factor of 9.77×10−9 (+8.79×10−8, −8.79×10−9) cm2 s−1 (95% confidence). The Ar solubility in labradorite was measured yielding a value of <0.2 ppb bar−1, which is similar to or lower than many rock forming minerals.

The labradorite diffusion parameters indicate Ar closure temperatures of 211 °C for a spherical diffusion geometry, and 243 °C for a planar diffusion geometry (for 100 µm-diameter grains, with cooling rates of 10 °C Ma−1). The data indicate that labradorite is less Ar retentive than K-feldspar at low temperatures, but more Ar retentive than K-feldspar at high temperatures, corroborating previous work on plagioclase. The relatively slow Ar diffusion rates in labradorite at magmatic temperatures may explain the common observation of older ages in large plagioclase grains in acidic volcanic systems.

Supplementary material:

Details of the UV laser depth profiles obtained from the labradorite and oligoclase samples used in this study are available at http://www.geolsoc.org.uk/SUP18608.

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Contents

Geological Society, London, Special Publications

Advances in 40Ar/39Ar Dating: From Archaeology to Planetary Sciences

F. Jourdan
F. Jourdan
Curtin University, Australia
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D. F. Mark
D. F. Mark
Scottish Universities Environmental Research Centre, UK
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C. Verati
C. Verati
University of Nice, France
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Geological Society of London
Volume
378
ISBN electronic:
9781862396623
Publication date:
January 01, 2014

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